The aquaculture supply chain in the time of covid-19 pandemic: Vulnerability, resilience, solutions and priorities at the global scale.

The COVID-19 global pandemic has had severe, unpredictable and synchronous impacts on all levels of perishable food supply chains (PFSC), across multiple sectors and spatial scales. Aquaculture plays a vital and rapidly expanding role in food security, in some cases overtaking wild caught fisheries in the production of high-quality animal protein in this PFSC. We performed a rapid global assessment to evaluate the effects of the COVID-19 pandemic and related emerging control measures on the aquaculture supply chain. Socio-economic effects of the pandemic were analysed by surveying the perceptions of stakeholders, who were asked to describe potential supply-side disruption, vulnerabilities and resilience patterns along the production pipeline with four main supply chain components: a) hatchery, b) production/processing, c) distribution/logistics and d) market. We also assessed different farming strategies, comparing land- vs. sea-based systems; extensive vs. intensive methods; and with and without integrated multi-trophic aquaculture, IMTA. In addition to evaluating levels and sources of economic distress, interviewees were asked to identify mitigation solutions adopted at local / internal (i.e., farm-site) scales, and to express their preference on national / external scale mitigation measures among a set of a priori options. Survey responses identified the potential causes of disruption, ripple effects, sources of food insecurity, and socio-economic conflicts. They also pointed to various levels of mitigation strategies. The collated evidence represents a first baseline useful to address future disaster-driven responses, to reinforce the resilience of the sector and to facilitate the design reconstruction plans and mitigation measures, such as financial aid strategies.

The effects of rearing density on growth, size heterogeneity and inter-individual variation of feed intake in monosex male Nile tilapia Oreochromis niloticus L.

The growth dispersion of farmed fish is a subject of increasing interest and one of the most important factors in stocking density. On a duration of 60 days, the effect of stocking density on the growth, coefficient of variation and inter-individual variation of feed intake (CVFI) of juvenile Nile tilapia Oreochromis niloticus L. (14.9 ± 1.2 g) were studied in an experimental tank-based flow-through system. Groups of fish were stocked at four stocking densities: 200, 400, 600 and 800 fish/m3, corresponding to a density of ∼3, 6, 9 and 12 kg/m3 and referred to as D1, D2, D3 and D4, respectively. Each treatment was applied to triplicate groups in a completely randomized design. No treatment-related mortality was observed. The fish densities increased throughout the experiment from 3 to 23.5, 6 to 43.6, 9 to 56.6 and 12 to 69 kg/m3. Results show that mass gain and specific growth rate (SGR, %M/day) were negatively correlated with increased stocking density. Groups of the D1 treatment reached a mean final body mass (FBM) of 119.3 g v. 88.9 g for the D4 groups. Feed conversion ratios (FCRs) were 1.38, 1.54, 1.62 and 1.91 at D1, D2, D3 and D4 treatments, respectively. Growth heterogeneity, expressed by the inter-individual variations of fish mass (CVM), was significantly affected by time (P < 0.001), stocking density (P < 0.001) and their interaction (P < 0.05). The difference in CVM was particularly conspicuous towards the end of the experiment and was positively correlated with stocking density. Similarly, radiographic study shows that CVFI was also found to be significantly greater for groups reared at high stocking densities (D3 and D4) than the other treatments (D1 and D2). These differences in both CVM and CVFI related to the stocking density need to be taken into account by husbandry practices to assure the production of more homogeneous fish size. A simple economic analysis indicates a parabolic relationship between profit and density with optimal final density at the peak of the curve. Given reasonable assumptions about production costs, the optimal final density (D opt) is 73.7 kg/m3. A sensitivity analysis shows that changes in the fixed cost have no effects on the optimal final density. However, small change in variable costs, such as feed and juvenile costs, may have substantial effect on the optimal density.

Size-dependent effects of daily thermal fluctuations on the growth and size heterogeneity of Nile tilapia Oreochromis niloticus.

The growth of Nile tilapia Oreochromis niloticus (0.02-20.00 g) was measured when fed to excess during the hours of light, following their exposure to five thermal regimes fluctuating around the thermal optimum for growth (T(opt) = 30 degrees C) over the diel cycle of day (light, L) and night (dark, N), i.e. 27 degrees C(L):33 degrees C(N), 28.5 degrees C(L):31.5 degrees C(N), 30 degrees C(L):30 degrees C(N), 31.5 degrees C(L):28.5 degrees C(N) and 33 degrees C(L):27 degrees C(N) (two replicates per treatment, six weeks' rearing, growth measurements at weekly intervals). A model constructed with a stepwise multiple-regression analysis accounted for 87.4% of the variation of the specific growth rate (G, % M day(-1)) from the variations of wet mass (M), the extent of the thermal fluctuation (F(T)) and their interactions, i.e. log(10)G = 1.7686 - 0.2136 log(10)M + 0.0806 [log (10)Mx log (10) (1 + F(T))] - 0.0394 [log(10)Mx log (10) (1 + F(T))](2). Based on this model, the thermal fluctuation that produces the fastest growth ( , degrees C) decreases in a curvilinear way, from 5.1 degrees C at 20 mg to c. 0.7 degrees C at 20 g. Thermal regimes that produce the slowest growth also produce the highest size heterogeneity. Functional hypotheses behind the size-dependent effects of thermal fluctuations are discussed, together with their implications in natural habitats and aquaculture systems with in different contexts of food availability.